Field of the Invention
The present disclosure relates to an optical element and a method for producing an optical element that includes a lens base and an antireflection film on a surface of the lens base, and to a method for producing an optical element.
Description of the Related Art
Optical elements, such as lenses, used for digital cameras and so forth usually include films having high optical transparency and low reflectance, i.e., antireflection films, arranged on their surfaces. To reduce the costs and weight of optical devices including optical elements, such as interchangeable lenses for digital cameras, resin lenses come into use instead of glass lenses. With the recent improvement of a technique for forming a resin lens, a demand for higher optical performance, and a demand for the miniaturization of an optical system, the use of large-curvature resin lenses each having a half aperture angle close to 45° has been studied.
A method for forming a multilayer film containing magnesium fluoride (MgF2), serving as a low-refractive-index material, with a refractive index of 1.38 by a vacuum deposition process is commonly known as a method for forming an antireflection film on a glass lens.
To increase the strength of a MgF2 film, however, a base needs to be heated to a high temperature (for example, about 300° C.) during the vapor deposition. This restricts the use of the MgF2 film for a resin lens. Thus, SiO2, serving as a low-refractive-index material that need not be heated, with a refractive index of 1.45 is commonly used as an antireflection film for a resin lens. However, SiO2 has a higher refractive index than MgF2 and thus has insufficient antireflection performance, disadvantageously leading to the formation of a ghost image.
Japanese Patent Laid-Open No. 2002-202401 discloses a technique for enabling the use of a MgF2 film for a resin lens, the technique including forming a MgF2 film on a film composed of an oxide of zirconium (Zr) or an oxide of titanium (Ti) by heating to a low temperature or without heating, and forming a SiO2 film serving as the outermost layer.
However, the structure of the antireflection film disclosed in Japanese Patent Laid-Open No. 2002-202401 has the problem that an antireflection film on a lens base is cracked when exposed to a high-temperature environment (for example, 70° C.) because a MgF2 film has a high tensile stress.